1. Field of the Invention
Embodiments of the present invention generally relate to a method and apparatus for manufacturing semiconductors, and more particularly to a method and apparatus for controlling the power output of a plasma source.
2. Description of the Related Art
Plasma processing is common in the semiconductor manufacturing industry. Plasma conditions are used to deposit, etch, clean, and dope substrates because ionized gases are more reactive, and thus create the materials needed for the applications more readily. In some applications, two electrodes are disposed adjacent to the reaction space so that an electric field may be generated therein. The electrodes act like a capacitor, and the electric field dissociates electrons from atoms and molecules between the electrodes. The resulting activated species react more readily than the stable electronically complete species. In other applications, a strong magnetic field may be used to generate the plasma. A gas may be forced through a tube around which an inductive coil is disposed. Current passing through the coil generates a strong magnetic field, and electrons bound to molecules moving through the field are energized to dissociate from those molecules. Alternately, the inductive coil may be disposed around a torroidal permanent magnet, which is itself disposed around the tube, to generate an inductively-coupled electric field inside the tube.
In a capacitatively-coupled plasma, the electric field is generated by charge separation between the two electrodes. In most cases, the charge separation is powered by an applied potential that oscillates at radio frequency. The substrate upon which species are to be deposited is placed on one of the electrodes. As charged species deposit on the surface, current flows, so power must be applied to maintain the charge separation. The amount of power that must be applied from moment to moment changes as the rate of deposition fluctuates. In most conventional capacitative applications, however, the power supply powering the electrode is not adjusted after it is initially set. When multiple deposition chambers are used, deposition rates and qualities among the chambers vary. Individual power supplies may also drift in the power they produce. Moreover, because plasmas are gases of charged particles, they can be unstable. A way is therefore needed to monitor and control rates of deposition in individual chambers directly in real-time.